An aggressive, or excited attitude characterized by its loudness. The term originates from electronic communication. Sentences and words spelled with the Caps Lock button turned on --in other words, all upper-case-- is the modern written equivalent of shouting at someone. It usually signifies anger, or sometimes intense excitement.

Kelly looked at Lisa incredulously. For a moment she was speechless. "WHAT THE HELL?! HOW COME YOU CAN'T MAKE IT TO JAKE'S BIRTHDAY PARTY?!" Lisa, startled by Kelly's shouting, replied, "Why you gotta get all caps lock on me?!"

Description: It is a button on the keyboard next to the 'a'.
Use: If pressed, it makes every letter that you type into all caps. Some people on the internet use it to emphasize a person's statement.

Caps Lock is th button on the keyboeard above it is the tab. The one below it is shift.
TOMMY THAT'S NOT NICE! That sentence would universally be translated as yelling, even without the exclamation point at the end.